(1) Field of the Invention
The invention described herein relates to hydrophone signal processing and in particular to digital demodulation for optical hydrophones.
(2) Description of the Prior Art
A typical optical hydrophone has a reference leg and a sensing leg. The sensing leg is formed by wrapping a fiber optic cable around a compliant mandrel. The reference leg is formed by wrapping a length of fiber optic cable around a noncompliant mandrel. During operation, light is pulsed down both fiber legs and reflected by mirrors imbedded in the ends of the fibers. The output of both legs, the reference and sensing legs, are summed at a node forming an interferometer. This summation produces a phase modulating signal of the form EQU O=A+Bcos.theta.(t) (1)
where
A & B=Constants proportional to the input power, and PA1 .theta.(t)=Phase difference between the interferometer sensor and reference leg. PA1 x(t)=Signal of interest, PA1 C=Modulating signal amplitude, and PA1 .omega..sub.0 =Modulating signal frequency.
Typically, a sinusoidal modulating frequency is injected through a piezoelectric element on the reference leg of the interferometer. The output signal is given by EQU O=A+Bcos(Ccos.omega..sub.0 (t)+x (t)) (2)
where
Typically, analog demodulators are used to process the output signal. These demodulators are complex custom-built hardware, requiring both expensive and time-consuming operation during calibration. What is needed is a system for using programmable digital signal processor for demodulation and for calibration.